1. Field of the Invention
The present invention relates to an apparatus and a method for controlling open/close timing of relay, especially to an apparatus and a method for switching a relay precisely at predetermined time points.
2. Description of Prior Art
As is well known, one use for relays is the delaying of signal connections to prevent a sudden current overload, generated by abrupt power switch on or switch off, from inputting into circuits of appliances. Relays are also used to connect to loads to cut out sudden large currents, caused by improper operation or control, for protecting the appliances from burning out.
Therefore, the relay is extensively used for switch control of electrical circuits. The relay generally comprises a coil and at least one contact controlled by the coil. More particularly, the contacts are switched between a close state and an open state in response to an energization and a de-energization of the coil.
Spark may be present when the contacts are switched with a load current. The contact resistance of the contact will be increased by the spark. Moreover, the contacts may be damaged earlier than expected by the spark. To ensure the lifetime of relay, the relay for alternative current operation is preferably switched at a zero-crossing point to prevent spark generation.
A time difference is inevitably present between the coil action and the contact opening/closing operation. The time difference between the coil action and the contact opening operation is referred to as an open time; and the time difference between the coil action and the contact closing operation is referred to as a close time. The open time and the close time may be the same or be different, and are generically referred to as responding time. The responding time can be known from the specification provided by manufacturer. However, the responding time may be drifted with operation time, temperature change and aging of the relay. Moreover, the responding time is also different for different manufacturer.
To switch the relay at zero-crossing point, relay controller such as a relay accelerator is developed to speed up the switch operation. However, the responding time cannot be minimized to zero and the zero-crossing point operation is still not realized.
U.S. Pat. No. 6,768,615 disclosed a relay controller with a memory unit. The responding time is pre-stored in the memory unit and the relay controller sends a control signal in advance to the zero-crossing point according to the pre-stored responding time for compensating the responding time. However, the relay controller relies on the specification provided by manufacturer and still cannot account for the practical factors such as operation time, temperature change and aging of the relay. For example, if the responding time is 5 ms according to the specification of manufacturer and is stored in the memory unit, the relay controller will generate the control signal with reference to the 5 ms responding time. However, the responding time may be changed to 10 ms due to above-mentioned practical factors, the zero-crossing operation still cannot be achieved.